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Engineering alloys can be conveniently subdivided into two types: ferrous and nonferrous. Ferrous alloys have iron as their principal alloying metal, whereas nonferrous alloys have a principal alloying metal other than iron. The steels, which are ferrous alloys, are by far the most important metal alloys, mainly because of their relatively low cost and wide range of mechanical properties. The mechanical properties of carbon steels can be varied considerably by cold working and annealing. When the carbon content of steels is increased to above about 0.3 percent, they can be heat-treated by quenching and tempering to produce high strength with reasonable ductility. Alloying elements such as nickel, chromium, and molybdenum are added to plain-carbon steels to produce low-alloy steels. Low-alloy steels have good combinations of high strength and toughness and are used extensively in the automotive industry for applications such as gears, shafts, and axles.

Aluminum alloys are the most important of the nonferrous alloys mainly because of their lightness, workability, corrosion resistance, and relatively low cost. Unalloyed copper is used extensively because of its high electrical conductivity, corrosion resistance, workability, and relatively low cost. Copper is alloyed with zinc to form a series of brass alloys that have higher strength than unalloyed copper.

Stainless steels are important ferrous alloys because of their high corrosion resistance in oxidizing environments. To make a stainless steel “stainless,” it must contain at least 12 percent Cr.

Cast irons are still another industrially important family of ferrous alloys. They are low in cost and have special properties such as good castability, wear resistance, and durability. Gray cast iron has high machinability and vibration-damping capacity due to the graphite flakes in its structure.

Other nonferrous alloys briefly discussed in this chapter are magnesium, titanium, and nickel alloys. Magnesium alloys are exceptionally light and have aerospace applications, and they are also used for materials-handling equipment. Titanium alloys are expensive but have a combination of strength and lightness not available from any other metal alloy system; they are used extensively for aircraft structural parts. Nickel alloys have high corrosion and oxidation resistance and are therefore commonly used in the oil and chemical process industries. Nickel when alloyed with chromium and cobalt forms the basis for the nickel-base superalloys needed in gas turbines for jet aircraft and some electric-power generating equipment.

In this chapter we have discussed to a limited extent the structure, properties, and applications of some of the important engineering alloys. We have also introduced special purpose alloys that are growing in importance and application in various industries. Of particular importance is the use of intermetallics, amorphous metals, and advanced superalloys in the biomedical field. These materials have superior properties to conventional alloys.







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